-1

我刚遇到以下情况：连接任意字符串和 NULL 值的 Integer 对象会导致 NullPointerException：

Integer nullInt = null;
String someString = "abc";

System.out.println(someString + "=" + nullInt)

现在我希望得到类似“abc=null”的结果，而不是 Java 在连接之前尝试拆箱 Integer 对象。为什么呢？

谢谢你的帮助！

更新

所以这是原始代码，我在上面的示例中对其进行了分解：

public class SomeClass {

   private String id;
   private String description;
   private Integer someInteger;
   private Date someDate;
   private Date someOtherInteger;

   // ...

   public String toString(){
       return "object [id=" + id
               + ", description=" + description
               + ", someInteger=" + someInteger
               + ", someDate=" + someDate
               + ", someOtherInteger=" + someOtherInteger;
   }

   // ...
}

"id" 有一个值 "description" 为空 "someInteger" 有一个值 "someDate" 为空 "someOtherInteger" 为空

罪魁祸首是“someOtherInteger”，我可以在 Eclipse 调试器中检查表达式“, someOtherInteger” + someOtherInteger 重现错误。确切的异常消息是：“java.lang.NullPointerException at intValue()”

有任何想法吗？

更新

这就是实际的代码，“lastMileage”是 NULL ......

@Override
public String toString(){
    return "OwnedCar [vin=" + vin
            + ", description=" + description
            + ", manufactureYear=" + manufactureYear
            + ", purchasedDate=" + purchasedDate
            + ", lastMileage=" + lastMileage
            + ", lastMileageDate=" + lastMileageDate
            + ", milesPerDay=" + milesPerDay
            + ", licencePlateNumber=" + licencePlateNumber
            + ", relationship=" + relationship
            + ", lifeCycleStage=" + lifeCycleStage   
            + ", lastKnownDeliveryStatus=" + lastKnownDeliveryStatus
            + ", lastKnownDeliveryStatusChangeDate=" + lastKnownDeliveryStatusChangeDate
            + ", lastDeliveryStatusCheckDate=" + lastDeliveryStatusCheckDate
            + ", orderCanceledCode=" + orderCanceledCode
            + ", deactivatedIndicator=" + deactivatedIndicator
            + ", deactivationReason=" + deactivationReason
            + ", modelCode=" + modelCode
            + ", model=" + model  
            + ", carAge=" + carAge                
            + ", vinVerifiedDate=" + vinVerifiedDate
            + ", commissionNumber=" + commissionNumber
            + ", commissionNumberYear=" + commissionNumberYear
            + ", ownershipActivationCode=" + ownershipActivationCode
            + ", bid=" + bid
            + ", orderNumber=" + orderNumber
            + ", largePhotoUrl=" + largePhotoUrl
            + ", mediumPhotoUrl=" + mediumPhotoUrl
            + ", smallPhotoUrl=" + smallPhotoUrl
            + ", colourSalesFamily =( "+ colourSalesFamily != null ? colourSalesFamily.toString() : null + ")"
            + "]";
}

这是堆栈跟踪：

java.lang.NullPointerException
    at ************.OwnedCar.toString(OwnedCar.java:671)
    at java.lang.String.valueOf(String.java:2826)
    at java.lang.StringBuilder.append(StringBuilder.java:115)
    at com.vw.dbs.******.***SessionAttributeListener.attributeReplaced(***SessionAttributeListener.java:46)
    at org.apache.catalina.session.StandardSession.setAttribute(StandardSession.java:1386)
    at org.apache.catalina.session.StandardSession.setAttribute(StandardSession.java:1291)
    at org.apache.catalina.session.StandardSessionFacade.setAttribute(StandardSessionFacade.java:130)
    at com.liferay.util.servlet.SharedSessionWrapper.setAttribute(SharedSessionWrapper.java:145)
    at com.liferay.portlet.PortletSessionImpl.setAttribute(PortletSessionImpl.java:273)
    at com.liferay.portlet.PortletSessionImpl.setAttribute(PortletSessionImpl.java:260)
    at org.springframework.web.portlet.context.PortletRequestAttributes.setAttribute(PortletRequestAttributes.java:156)
    at org.springframework.web.bind.support.DefaultSessionAttributeStore.storeAttribute(DefaultSessionAttributeStore.java:54)
    at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.updateModelAttributes(HandlerMethodInvoker.java:214)
    at org.springframework.web.portlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:363)
    at org.springframework.web.portlet.mvc.annotation.AnnotationMethodHandlerAdapter.doHandle(AnnotationMethodHandlerAdapter.java:347)
    at org.springframework.web.portlet.mvc.annotation.AnnotationMethodHandlerAdapter.handleRender(AnnotationMethodHandlerAdapter.java:289)
    at org.springframework.web.portlet.DispatcherPortlet.doRenderService(DispatcherPortlet.java:734)
    at org.springframework.web.portlet.FrameworkPortlet.processRequest(FrameworkPortlet.java:522)
    at org.springframework.web.portlet.FrameworkPortlet.doDispatch(FrameworkPortlet.java:470)
    at javax.portlet.GenericPortlet.render(GenericPortlet.java:233)
    at com.liferay.portlet.FilterChainImpl.doFilter(FilterChainImpl.java:101)
    at com.liferay.portal.kernel.portlet.PortletFilterUtil.doFilter(PortletFilterUtil.java:64)
    at com.liferay.portal.kernel.servlet.PortletServlet.service(PortletServlet.java:92)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:551)
    at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:488)
    at com.liferay.portlet.InvokerPortletImpl.invoke(InvokerPortletImpl.java:638)
    at com.liferay.portlet.InvokerPortletImpl.invokeRender(InvokerPortletImpl.java:723)
    at com.liferay.portlet.InvokerPortletImpl.render(InvokerPortletImpl.java:425)
    at org.apache.jsp.html.portal.render_005fportlet_jsp._jspService(render_005fportlet_jsp.java:1440)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:551)
    at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:488)
    at com.liferay.portal.util.PortalImpl.renderPortlet(PortalImpl.java:3715)
    at com.liferay.portal.util.PortalUtil.renderPortlet(PortalUtil.java:1180)
    at com.liferay.portlet.layoutconfiguration.util.RuntimePortletUtil.processPortlet(RuntimePortletUtil.java:160)
    at com.liferay.portlet.layoutconfiguration.util.RuntimePortletUtil.processPortlet(RuntimePortletUtil.java:94)
    at com.liferay.portlet.layoutconfiguration.util.RuntimePortletUtil.processTemplate(RuntimePortletUtil.java:256)
    at com.liferay.portlet.layoutconfiguration.util.RuntimePortletUtil.processTemplate(RuntimePortletUtil.java:181)
    at org.apache.jsp.html.portal.layout.view.portlet_jsp._jspService(portlet_jsp.java:821)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.doInclude(ApplicationDispatcher.java:551)
    at org.apache.catalina.core.ApplicationDispatcher.include(ApplicationDispatcher.java:488)
    at com.liferay.portal.action.LayoutAction.includeLayoutContent(LayoutAction.java:361)
    at com.liferay.portal.action.LayoutAction.processLayout(LayoutAction.java:618)
    at com.liferay.portal.action.LayoutAction.execute(LayoutAction.java:229)
    at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:431)
    at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:236)
    at com.liferay.portal.struts.PortalRequestProcessor.process(PortalRequestProcessor.java:152)
    at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
    at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    at com.liferay.portal.servlet.MainServlet.callParentService(MainServlet.java:518)
    at com.liferay.portal.servlet.MainServlet.service(MainServlet.java:495)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.strip.StripFilter.processFilter(StripFilter.java:309)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.themepreview.ThemePreviewFilter.processFilter(ThemePreviewFilter.java:88)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.gzip.GZipFilter.processFilter(GZipFilter.java:137)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.secure.SecureFilter.processFilter(SecureFilter.java:182)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.autologin.AutoLoginFilter.processFilter(AutoLoginFilter.java:254)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
    at com.liferay.portal.servlet.FriendlyURLServlet.service(FriendlyURLServlet.java:133)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.strip.StripFilter.processFilter(StripFilter.java:261)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.themepreview.ThemePreviewFilter.processFilter(ThemePreviewFilter.java:88)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.gzip.GZipFilter.processFilter(GZipFilter.java:126)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.secure.SecureFilter.processFilter(SecureFilter.java:182)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.i18n.I18nFilter.processFilter(I18nFilter.java:211)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.autologin.AutoLoginFilter.processFilter(AutoLoginFilter.java:254)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
    at com.liferay.portal.servlet.I18nServlet.service(I18nServlet.java:101)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
    at com.liferay.portal.servlet.filters.virtualhost.VirtualHostFilter.processFilter(VirtualHostFilter.java:310)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:126)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:738)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at com.liferay.portal.kernel.servlet.BaseFilter.processFilter(BaseFilter.java:196)
    at com.liferay.portal.servlet.filters.threadlocal.ThreadLocalFilter.processFilter(ThreadLocalFilter.java:35)
    at com.liferay.portal.kernel.servlet.BaseFilter.doFilter(BaseFilter.java:123)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:470)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:555)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
    at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:190)
    at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:291)
    at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:774)
    at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:703)
    at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:896)
    at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:690)
    at java.lang.Thread.run(Thread.java:662)
4

3 回答 3

5

并非如此：

public static void main(String[] args) {
    Integer nullInt = null;
    String someString = "abc";

    System.out.println(someString + "=" + nullInt);
}

输出：

abc=null
于 2012-04-10T17:23:37.477 回答
1

我跑你的台词，它写着我abc=null。

于 2012-04-10T17:23:36.737 回答
1

JLS 15.18.1.1（字符串与+操作数连接）：

如果引用为 null，则将其转换为字符串“null”（四个 ASCII 字符 n、u、l、l）。否则，转换就像调用被引用对象的 toString 方法一样执行，不带参数；但如果调用 toString 方法的结果为 null，则使用字符串“null”代替

所以下面的代码

String a = "abc=" + anInteger;

从不抛出异常（abc=null虽然它可能会写）但是如果你对它执行一些算术：

String a = "abc=" + (anInteger + 1)

anInteger如果null在拆箱时，它显然会在 rutime 抛出 NPE 。也许你没有发布你的整个代码

更新

OP更新了他的答案，原因需要进一步解释

public class Main {

    public static void main(String[] args) {
        Integer i = null;
        System.out.println("throw a NPE " + i != null ? i.toString() : null);
    }
}

三元运算符周围没有括号，因此表达式被计算为

System.out.println(("throw a NPE " + i) != null ? i.toString() : null);

首先应用字符串连接，因此“throw a NPE null”不是null并且i.toString()被调用，抛出 NPE

于 2012-04-10T19:04:29.817 回答